1. Field of the Invention
In general, this invention relates to a method and apparatus for continuously or semicontinuously melting and refining metal in a vertical shaft furnace by contacting the metal pieces with heat produced by the combustion of a fuel mixture having a carefully controlled composition which is burned and injected into the furnace through a plurality of burner openings in the refractory brick furnace wall. Melted metal drains to the bottom of the furnace and flows out of the furnace through a tapping outlet in a substantially continuous stream for further processing.
2. Description of the Prior Art
Conventional vertical shaft furnaces for melting and refining metal, such as copper, are well known; and may hearth arrangements have been used in them for various reasons. Such vertical furnaces and hearth arrangements are disclosed in U.S. Pat. Nos. 2,283,163, 3,199,977, 3,715,203, 3,788,623 and in some of the prior art cited in these. Generally, these furnaces have a substantially cylindrical shape and are elongated in a vertical direction. The metal to be melted, such as copper cathode pieces, is charged into the furnace from an elevated position. These charge pieces drop toward the bottom of the furnace where a plurality of burners located in the walls of the furnace inject heat into the furnace to cause the metal to melt. The molten metal is continuously drained from the furnace through a suitable tapping outlet in the bottom of the furnace and then usually passes to a holding furnace or to a further process such as a continuous casting operation.
Refractory walls of a conventional vertical shaft furnace typically increase in thickness from the upper portion of the furnace to the lower portion of the furnace. This often forms a funnel-shaped interior melting chamber extending from the charge entrance to an area adjacent to the top of the hearth. The funnel-shaped refractory provides continuing support for charge pieces as they melt, shrink and travel down the shaft of the furnace. This support usually works very well for normally large charge pieces such as copper cathodes. However, when the charge is initially smaller than the smallest portion of the funnel; such as copper scrap, blister cakes, spent anodes and the like; or when the charge pieces shrink to a smaller size during continuous melting, and particularly during problem periods which cause repeated startups and shutdowns, the funnel support method is ineffective. The result is unmelted metal pieces resting directly on the hearth, which can easily lead to several problems in the melting and refining process.
One problem encountered with prior art vertical shaft furnaces and their hearth arrangements is that burner openings through which heat enters the furnace can become substantially blocked by the pieces of metal which reach the hearth before they are completely melted. A blocked burner may misdirect heat which will result in inefficient melting and may lead to further blockage. Blocked burners may even blackfire, which often causes malfunctions and damage to the burners and adjacent parts. If pieces of metal mechanically blocks off a burner, incomplete combustion may result. This is considered particularly undesirable because, especially in molten copper, oxygen content is critical. Since the burner ignites a mixture of fuel and oxygen-containing gases, incomplete combustion results in excess oxygen being injected into the furnace and oxygen content in the molten metal could therefore be raised above desirable or acceptable limits. In addition, stagnant pools of molten metal may accumulate allowing formation or concentration of slag, which even if quickly corrected causes poor quality products because slag particles cannot be evenly dispersed again. In extreme cases, shut down, cold cleaning and rebuilding of the furnace may be required.
These problems, and the complications stemming from them, cause significant economic loss because of poor quality copper produced, impaired production, and excessive expenditures of time and money to repair clogged and damaged furnaces.